The present invention relates to a thermal printer which forms a letter or character image on a thermosensitive sheet.
Conventionally, a thermal printer forms an image on a thermosensitive sheet by using a thermal head having a plurality of thermal elements which applies heat to the thermosensitive sheet. The thermosensitive layer on the thermosensitive sheet is heated and developed (colored) when a certain heat energy is applied. The darkness of the color depends on the quantity of the applied energy. More specifically, the darkness of the color is related to the density of the thermosensitive pigment in the thermosensitive layer, that undergoes a change in color when heat is applied to the thermosensitive sheet. However, in such a printer, if the voltage of the battery changes during a printing operation, the darkness of the print will not be uniform, and a poor quality printout will result.
Further, in such a printer, the voltage to be applied to the thermal elements is determined to be in a predetermined range. Thus, if a voltage out of the range is applied to the thermal elements, for example, through a connector for an external power source, then the print may be too dark or too light.